Dry-distilled coal has an activated surface, which tends to bond with oxygen. Accordingly, when the coal is stored as it is, heat generated by reaction with oxygen in air may cause the coal to spontaneously combust. In view of this, oxygen is first bonded to the surface of the dry-distilled coal by exposing the coal to a processing gas atmosphere containing oxygen and the coal is thereby deactivated. The spontaneous combustion in storage is thus prevented.